With the increasing concern of society in protecting and improving the environment, the problem of noise pollution is receiving much study. People who live in cities are surrounded by all manner and variety of noises ranging from the roar of jet planes to the rumble of freeway traffic. This vast accumulation of noise can be quite harmful in causing diminished hearing. Also, although not precisely measurable, noise can and does have an adverse effect upon the emotional stability and mental health of the recipients.
People are now asking seriously whether machines created to be servants of man are not, in fact, becoming his master. Surely, this is a fair question in regard to noise pollution where the constant cacophony of machine noises disrupts man's thinking processes, interferes with his sleep, and oftentimes threatens to destroy his sanity.
A common source of noise in the city is the strident noise of a truck engine as it turns over at high speeds in operating the auxiliary equipment carried on the truck. As one example, consider the noise of a truck engine in operating the packing mechanism for refuse collection equipment during the pick up of refuse in a residential neighborhood. The collection of refuse begins early in the morning and, thus, the noise of the laboring truck engine operating at high speed may well be the first sound which greets the ear of a city resident on awakening. Even worse, the noise of the truck engine may be the causation for an unwanted early awakening by the sleeper.
Many types of auxiliary truck equipment, such as the packing mechanism for a refuse loader, go through a cycling operation in which the load requirements on the engine will vary. Thus, for example, in the operation of a refuse packing mechanism, the refuse will first be placed in a hoper positioned within a tailgate structure on a truck. The refuse is then swept from the hopper and moved through an opening connecting the hopper with a refuse storage body positioned on the truck frame. As the refuse is moved into the opening and packed under great pressures, the pressure demands of the packing mechanism are very high. The pressure demands at this stage of packing are generally considerably higher, for example, than the pressure demands when the refuse is being swept from the hopper. For a more detailed description of a refuse packing mechanism, reference is made to U.S. Pat. No. 2,879,906, issued Mar. 31, 1959.
During operation of an auxiliary load on a truck, it has previously been necessary to maintain the truck engine at a relatively high speed when the auxiliary load is variable, as in the case of a refuse packing mechanism. This was done to prevent the truck engine from stalling when the power demands imposed by the variable load were increased. To prevent stalling, it was customary to maintain the truck engine at a relatively high speed to provide the output from the engine at a level sufficient to operate the variable load under the maximum power conditions imposed by the load. This method of operation, while perhaps satisfactory to prevent stalling, was inefficient and produced a high noise level from the truck engine in driving the auxiliary load.
By maintaining the engine speed at a level to supply the maximum power demand from the auxiliary load, the engine produced more power than was needed by the load at power demands less than its maximum. To dissipate this unused power during the minimum power demand portions of the variable load cycle, it was necessary to shunt a portion of the output from a pump driven by the engine to a sump during the minimum power demand portions of the variable load cycle. Thus, the full output of the pump was used only during the maximum power demand portions of the variable load cycle.
In the operation of auxiliary equipment, some manufacturers have mounted an auxiliary engine on the truck to operate the auxiliary equipment. The auxiliary engines are relatively small and operate at a high rate of speed to develop their maximum horse-power in driving the auxiliary equipment. This type of operation has not been satisfactory in terms of abating noise pollution since a small engine operating at a high speed is a very efficient generator of noise. Also, this type of operation is inefficient since it requires dissipation of unusable engine output except when the load is in the maximum power demand portion of its cycle.